1. Field of the Invention
The present invention relates to a digital radio communication system, especially to a mobile communication terminal in a mobile communication system, and more particularly to an antenna weight estimation method in a system to which the transmit diversity utilizing a plurality of antennas in a radio base station is applied.
The present invention supposes the system that employs the closed loop mode 1 described in the document 3GPP RAN TS25.214 V3.1.0, December 1999 as the transmit diversity utilizing base station antennas. Although the present invention is applicable to radio access schemes other than CDMA (Code Division Multiple Access), since the foregoing document applies the transmit diversity to a CDMA system, the following description also supposes the CDMA as the radio access scheme.
2. Description of the Related Art
The transmit diversity based on the closed loop mode 1 is a system using two base station antennas. FIG. 1 shows an outline of the transmit diversity utilizing the closed loop mode 1. A downlink individual channel to which the closed loop mode 1 is applied is multiplied by antenna weights, and then multiplexed with other channels by channel multiplexers 11a and 11b, followed by filtering and frequency conversion by transmitting radio sections 12a and 12b. The antenna weights are controlled by an antenna weight control command, which is mapped onto an uplink individual channel, such that the received signal power increases in a mobile communication terminal. The antenna weight control command is called feedback information (abbreviated to FBI from now on), and a FBI decoder 13 calculates the antenna weights from the FBI mapped onto the uplink individual channel.
It is necessary for the mobile communication terminal to estimate the antenna weights used for the received signals to carry out the Rake combining of the received signals, or to estimate a received SNIR (signal noise interference power ratio). If the base station uses the antenna weights designated by the mobile communication terminal without error, and the mobile communication terminal knows the timing at which the FBI command it designates is reflected in the base station, the mobile communication terminal can decide the antenna weights with believing the FBI command it transmits. Usually, however, the transmitted FBI is not always reflected in the base station correctly because of bit errors in the uplink FBI command. In view of this, the mobile communication terminal estimates the antenna weights using, besides the FBI command, the received signals of individual pilot symbols contained in the individual channels whose quadrature patterns are transmitted from the antennas.
The foregoing document describes an example of the antenna weight estimation method (Simplified Beam Former Verification). This method estimates the transmission antenna weights as follows, where d is an antenna weight control delay in terms of slots, and i is a received slot number in which the uplink FBI bits in the (i-d)th slot are reflected onto the downlink antenna weight.
<When i-d is an Even Number>
The I-component x0 of the antenna weight is estimated by the following algorithm.If                               2          ⁢                                    ∑                              k                =                0                            K                        ⁢                                          1                                                      σ                    ~                                    k                  2                                            ⁢                              {                                  2                  ⁢                                      Re                    ⁡                                          (                                                                        γ                          ~                                                ⁢                                                                                                            c                              ~                                                                                      d                              ,                              2                              ,                              k                                                                                ·                                                                                    c                              ~                                                                                      p                              ,                              2                              ,                              k                                                        *                                                                                              )                                                                      }                                                    >                  ln          ⁡                      (                                                            p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                    π                                    )                                                                              p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                    0                                    )                                                      )                                              (        1        )            then x0=0  (2)elsex0=π  (3)<When i-d is an Odd Number>
The Q-component x1 of the antenna weight is estimated by the following algorithm.             If                                                                             -            2                    ⁢                                    ∑                              k                =                0                            K                        ⁢                                          1                                                      σ                    ~                                    k                  2                                            ⁢                              {                                  2                  ⁢                                      Im                    ⁡                                          (                                                                        γ                          ~                                                ⁢                                                                                                            c                              ~                                                                                      d                              ,                              2                              ,                              k                                                                                ·                                                                                    c                              ~                                                                                      p                              ,                              2                              ,                              k                                                        *                                                                                              )                                                                      }                                                    >                  ln          ⁡                      (                                                            p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                                          -                                              π                        2                                                                              )                                                                              p                  _                                ⁡                                  (                                                            ϕ                      Rx                                        =                                          π                      2                                                        )                                                      )                                              (        4        )                        then                                                                 x          1                =                  -                      π            2                                              (        5        )                        else                                                                 x          1                =                  π          2                                    (        6        )            where {tilde over (c)}p,2,k(n) is a channel estimation value of the kth finger of the antenna 2 obtained by using a common pilot channel, {tilde over (c)}d,2,k(n) is a channel estimation value (including the antenna weight) of the kth finger of the antenna 2 obtained by using an individual pilot channel, {tilde over (σ)}k2 is an estimation value of the noise interference power of the kth finger, {tilde over (γ)}2 is an estimation value of the ratio between the received power of the individual pilot symbol and the received signal power of the common pilot channel, and {overscore (p)}(φRx=x) is the prior probability that the phase difference becomes x, where the phase difference is that between the I-component or Q-component of the antenna weight obtained from the FBI bit the mobile communication terminal itself transmit and the estimated error rate of the FBI bit. The X0 and X1 keep their value until updated, and the antenna weight estimation value {tilde over (w)} is given by                               w          ~                =                                                            ∑                                  i                  =                  0                                1                            ⁢                              cos                ⁡                                  (                                      x                    i                                    )                                                                    2                                +                      j            ⁢                                                   ⁢                                                            ∑                                      i                    =                    0                                    1                                ⁢                                  sin                  ⁡                                      (                                          x                      i                                        )                                                                              2                                                                        (        7        )            
This method does not estimate one of the foregoing four states (π/4, 3π/4, −3π/4, −π/4) the antenna weight can take from the individual pilots received by the antenna 2 in each slot, but estimates, as to the FBI the base station receives in each slot, the phase difference that is updated in each slot by using the FBI the mobile communication terminal itself transmits and the individual pilots received by the antenna 2.
FIG. 2 illustrates values and estimation timings the phase difference of the antenna weight can take. It is shown from FIG. 2 that the I-components of the weight are estimated in the received slots #0+d, 2+d, 4+d, 6+d, 8+d, 10+d, 12+d and 14+d, and the Q-components are estimated in the remaining received slots.
Since the conventional antenna weight estimation method described above estimates only the timings of change of the phase difference by the FBI, it cannot carry out the estimation by making full use of the received power of the individual pilots fed from the antenna 2. More specifically, although the I-components of the antenna weight selected in even numbered slot are used until the next odd numbered slot arrives and the Q-components of the antenna weight selected in the odd numbered slot are used until the next even numbered slot arrives, the conventional method, which estimates the weight only immediately after the components are changed, does not use the received signals of the individual pilots received by the antenna 2 in the following slots.
To make better use of the received signals of the individual pilot symbols for estimating the antenna weight, a method is effective that estimates the I-components of the weight using, after receiving an odd numbered slot, the individual pilots in both the previous even numbered slot and in the odd numbered slot, and estimates the Q-components of the weight using, after receiving an even numbered slot, the individual pilots in both the previous odd numbered slot and in the even numbered slot. However, as for the antenna weight used for estimating the received SNIR for the transmission power control, it must be estimated instantaneously at the timings when the individual pilots are received.